gpiolib: use gpiochip_dup_line_label() in for_each helpers

Rework for_each_requested_gpio_in_range() to use the new helper to
retrieve a dynamically allocated copy of the descriptor label and free
it at the end of each iteration. We need to leverage the CLASS()'
destructor to make sure that the label is freed even when breaking out
of the loop.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>